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How do I rescan the SCSI bus to add or remove a SCSI device without rebooting the computer?

 Updated 19 Nov 2012, 7:29 PM GMT

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Issue

·         It is possible to add or remove a SCSI device without rebooting a running system?

·         Can you scan a SCSI bus for new or missing SCSI devices without rebooting?

·         What is the Linux equivalent to the Solaris command `devfsadm` to add or remove storage devices?

·         How can I make newly connected SCSI devices available without rebooting?

·         I am trying to add a LUN to a live system but it is not recognized

·         How can I force a rescan of my SAN?

·         What to do if a newly allocated LUN on my SAN is not available?

Environment

·         Red Hat Enterprise Linux 5.0 or above   

o    SCSI devices over a Fibre Channel or iSCSI transport

Technical support for online storage reconfiguration is provided on Red Hat Enterprise Linux 5 and above. Limited tools for hot adding and removing storage are present in previous releases of Red Hat Enterprise Linux however they cannot be guaranteed to work correctly in all configurations.  Red Hat Enterprise Linux 5 includes many enhancements to udev, the low level device drivers, SCSI midlayer, and device-mapper multipath which enables comprehensive support for online storage reconfiguration.

This article, the Online Storage Reconfiguration Guide, and the Storage Administration Guide currently cover the FC and iSCSI transports. Future versions of this documentation will cover other SCSI transports, such as SAS and FCoE.

Hewlett-Packard SmartArray controllers and other hardware that uses the cciss driver provide a different interface for manipulating SCSI devices.  Users of this hardware can find a similar guide here.

The procedures below also apply to hypervisors (i.e. "dom0" in Red Hat Enterprise Linux 5 virtualization), but the procedures are different for dynamically altering the storage of running virtual guests. For more information about adding storage to virtual guests, see the Virtualization Guide.

Resolution

Yes, as of Red Hat Enterprise Linux 5.0, it is possible to make changes to the SCSI I/O subsystem without rebooting. There are a number of methods that can be used to accomplish this, some perform changes explicitly, one device at a time, or one bus at a time. Others are potentially more disruptive, causing bus resets, or potentially causing a large number of configuration changes at the same time. If the less-disruptive methods are used, then it is not necessary to pause I/O while the change is being made. If one of the more disruptive methods are used then, as a precaution, it is necessary to pause I/O on each of the SCSI busses that are involved in the change.

This article is a brief summary of the information contained in the Red Hat Enterprise Linux manuals. For Red Hat Enterprise Linux 5 refer to Online Storage Reconfiguration Guide. For Red Hat Enterprise Linux 6 refer to Storage Administration Guide.   You must refer to these documents for complete coverage of this topic.

Removing a Storage Device

Before removing access to the storage device itself, you may want to copy data from the device. When that is done, then you must stop and flush all I/O, and remove all operating system references to the device, as described below.  If this is a multipath device then you must do this for the multipath pseudo device, and each of the identifiers that represent a path to the device.

Removal of a storage device is not recommended when the system is under memory pressure, since the I/O flush will add to the load. To determine the level of memory pressure run the command:

vmstat 1 100

Device removal is not recommended if swapping is active (non-zero "si" and "so" columns in the vmstat output), and free memory is less than 5% of the total memory in more than 10 samples per 100.  (The total memory can be obtained with the "free" command.)

The general procedure for removing all access to a device is as follows:

      1. Close all users of the device. Copy data from the device, as needed.

      2. Use umount to unmount any file systems that mounted the device.

      3. Remove the device from any md and LVM volume that is using it. If the device is a member of an LVM Volume group, then it may be necessary to move data off the device using the pvmove command, then use the vgreduce command to remove the physical volume, and (optionally) pvremove to remove the LVM metadata from the disk.

      4. If you are removing a multipath device, run multipath -l and take note of all the paths to the device. When this has been done, remove the multipath device:

multipath -f multipath-device

Where multipath-device is the name of the multipath device mpath0, for example.

NOTE: This command may fail with "map in use" if the multipath device is still in use (for example, a partition is on the device).  Seehttps://access.redhat.com/kb/docs/DOC-56916 for further details.

      5. Use the following command to flush any outstanding I/O to all paths to the device:

blockdev --flushbufs device

      This is particularly important for raw devices, where there is no umount or vgreduce operation to cause an I/O flush.

      6. Remove any reference to the device's path-based name, like /dev/sd or /dev/disk/by-path or the major:minor number, in applications, scripts, or utilities on the system.  This is important to ensure that a different device, when added in the future, will not be mistaken for the current device.

      7. The final step is to remove each path to the device from the SCSI subsystem.  The command to remove a path is:

echo 1 >  /sys/block/device-name/device/delete

Where device-name may be sde, for example.

Another variation of this operation is:

echo 1 >  /sys/class/scsi_device/h:c:t:l/device/delete

Where h is the HBA number, c is the channel on the HBA, t is the SCSI target ID, and l is the LUN.

You can determine the device-name and the h,c,t,l for a device from various commands, such as lsscsi, scsi_id, multipath -l, andls -l /dev/disk/by-*

If each of the steps above are followed, then a device can safely be removed from a running system. It is not necessary to stop I/O to other devices while this is done.

Other procedures, such as the physical removal of the device, followed by a rescan of the SCSI bus using rescan-scsi-bus or issue_lip to cause the operating system state to be updated to reflect the change, are not recommended. This may cause delays due to I/O timeouts, and devices may be removed/replaced unexpectedly. If it is necessary to perform a rescan of an interconnect, it must be done while I/O is paused. Refer to Online Storage Reconfiguration Guide and  Storage Administration Guide for more information.

Adding a Storage Device or a Path

When adding a device, be aware that the path-based device name (the “sd” name, the major:minor number, and /dev/disk/by-path name, for example) that the system assigns to the new device may have been previously in use by a device that has since been removed. Ensure that all old references to the path-based device name have been removed. Otherwise the new device may be mistaken for the old device.

The first step is to physically enable access to the new storage device, or a new path to an existing device.  This may involve installing cables, disks, and vendor-specific commands at the FC or iSCSI storage server. When you do this, take note of the LUN value for the new storage that will be presented to your host.

Next, make the operating system aware of the new storage device, or path to an existing device. The preferred command is:

echo "c t l" >  /sys/class/scsi_host/hostH/scan

where H is the HBA number, c is the channel on the HBA, t is the SCSI target ID, and l is the LUN.

You can determine the H,c,t by refering to another device that is already configured on the same path as the new device. This can be done with commands such as lsscsi, scsi_id, multipath -l, and ls -l /dev/disk/by-*. This information, plus the LUN number of the new device, can be used as shown above to probe and configure that path to the new device.

Note: In some Fibre Channel hardware configurations, when a new LUN is created on the RAID array it may not be visible to the operating system until after a LIP (Loop Initialization Protocol) operation is performed. Refer to the manuals for instructions on how to do this. If a LIP is required, it will be necessary to stop I/O while this operation is done.

As of Red Hat Enterprise Linux 5.6, it is also possible to use the wildcard character "-" in place of c, t and/or l in the command shown above. In this case, it is not necessary to stop I/O  while this command executes. In versions prior to 5.6, the use of wildcards  in this command requires that I/O be paused as a precaution.

After adding all the SCSI paths to the device, execute the multipath command, and check to see that the device has been properly configured. At this point, the device is available to be added to md, LVM, mkfs, or mount, for example.

Other commands, that cause a SCSI bus reset, LIP, or a system-wide rescan, that may result in multiple add/remove/replace operations, are not recommended. If these commands are used, then I/O to the effected SCSI buses must be paused and flushed prior to the operation. Refer to the Online Storage Reconfiguration Guide and Storage Administration Guide for more information.

As of release 5.4, a script called /usr/bin/rescan-scsi-bus.sh is available as part of the sg3_utils package. This can make rescan operations easier. This script is described in the manuals mentioned above.`

 


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